Generally, there have been known so-called electrostatic coating apparatuses which are comprised of a coating machine employing a rotary atomizing head to spray paint toward a coating object, a high voltage generator boosting a power supply voltage for generating a high voltage and outputting the high voltage to the rotary atomizing head of the coating machine, a power supply voltage control unit controlling a power supply voltage to be supplied to the high voltage generator and a high voltage control unit outputting a setting signal to the power supply voltage control unit to designate a power supply voltage and controlling a high voltage to be output by the high voltage generator (see, for example, Japanese Patent Laid-Open No. 2002-186884).
According to the electrostatic coating apparatus provided by the prior arts, the rotary atomizing head serves as an electrode for discharging a high voltage toward a coating object. Therefore, an electrostatic field is formed between the rotary atomizing head and the coating object being a ground potential. Moreover, paint particles charged at a high voltage through the rotary atomizing head are flied along the electrostatic field to the coating object and land thereon.
Further, in the electrostatic coating apparatus, the low voltage side of the high voltage generator is maintained as the ground potential. Therefore, for the electrostatic coating apparatus, an electrostatic field is formed not only between the rotary atomizing head and the coating object as described above, but also between the rear side of the electrostatic coating apparatus which is the ground side of the high voltage generator and the rotary atomizing head. At this time, suspended particles such as a sprayed mist and dust, water in the air and so forth are adsorbed and attached to the surface of the cover of the coating machine, and it effects to reduce the surface resistance of the cover and deteriorate the insulation of the electrostatic coating apparatus. Now, a high voltage application path is formed by the paths of the power supply, the high voltage generator, the rotary atomizing head, the coating object, and so forth. In the case of the electrostatic coating apparatus according to the above-mentioned prior art, a current (hereinafter called a full return current) that flows through the path of the high voltage generator contained in the high voltage application path is detected, and based on the amplitude of the detected current, deterioration of the insulation of the cover is detected.
In the case of the electrostatic coating apparatus by the above-mentioned prior art, deterioration of the insulation of the cover is detected based on the full return current that flows through the high voltage generator that forms part of the high voltage application path. However, in addition to a current (hereafter referred to as an object current) that flows between the rotary atomizing head and the coating object along the high voltage application path, there is also a current (hereinafter referred to as a leakage current) that flows along a leakage path other than the high voltage application path while also passing through the high voltage generator. Therefore, the full return current includes the object current that passes between the rotary atomizing head and the coating object, and the leakage current that flows along the surface of the coating machine. At this time, the leakage current of the coating machine occurs are not only the surface of the cover of the coating machine, but also the inner wall of the paint passage in the coating machine and the inner wall of the air passage for spray pattern formation, and so forth.
For example, even if the inner wall of the paint passage is appropriately cleansed, pigments contained in the paint tend to gradually accumulate as the operation is continued. Therefore, due to the residually accumulated pigments, the insulation resistance is reduced and a high voltage creepage discharge tends to occur. Especially when a so-called metallic paint containing a metal pigment such as aluminum powder is employed, the pigment served as a conductor accumulates on the inner wall of the paint passage, so that a reduction in the insulation resistance becomes noticeable.
Furthermore, when shaping air for spray pattern formation, pilot air for an air valve to control the supply of paint and the cutoff of the supply, and drive air for an air motor to drive the rotary atomizing head, are passed along the air passage, fine dust and water contained in the air are deposited to the inner wall of this passage and a high voltage creepage discharge tends to occur.
As described above, the coating machine is in a state wherein a leakage current could occur at plural positions. On the other hand, when a reduction of the insulation based on the full return current is detected it is difficult to determine either the object current or the leakage current are increased, and furthermore the location occurred the leakage current can not be identified.
Thus, the leakage current can not be sufficiently prevented by cleaning the surface of the cover of the coating machine and a cutoff of the high voltage frequently occurs due to an increase in an abnormal current value, so that stop times of the coating machine tends to be increased and the coating productivity is lowered. In addition, since the location occurred the leakage current can not be identified, a progress of a dielectric breakdown for the surface of the cover, the inner wall of the paint passage and the air passage are unknown, and damage (electric damage-by-fire) to the coating machine can not be prevented.